Hydraulic fracturing is used to create subterranean fractures that extend from the borehole into the rock formation to improve the rate of production. Generally, a high viscosity fracturing fluid is pumped into the well at sufficient pressure to fracture the subterranean formation. In order to maintain the increased exposure of the formation, a solid proppant may be added to the fracturing fluid, which is carried into the fracture by the high pressure applied to the fluid.
Some conventional fracturing fluids include guar gum (galactomannans) or guar gum derivatives, such as hydroxypropyl guar (HPG), carboxymethyl guar (CMG), or carboxymethylhydroxypropyl guar (CMHPG). These polymers can be crosslinked in order to increase their viscosities and increase their capabilities of proppant transport.
Once the formation is adequately fractured and the proppant is in place, the fracturing fluid is recovered typically through the use of breakers. Breakers generally reduce the fluid's viscosity to a low enough value allowing the proppant to settle into the fracture, thereby exposing the formation to the well. Breakers work by severing the bonds of the polymer. This “breaks” the polymer reducing its molecular weight. The fracture then becomes a high permeability conduit for fluids and gas to be produced back to the well.
Breakers control the timing of the break for the fracturing fluid. Gels that break prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Also, premature breaking can result in a premature reduction in the fluid viscosity resulting in a less than desirable fracture length in the fracture being created. For purposes of the present application, premature breaking will be understood to mean that the gel viscosity becomes diminished to an undesirable extent before all of the fluid is introduced into the formation to be fractured.
On the other hand, gelled fluids that break too slowly can cause slow recovery of the fracturing fluid and a delay in resuming the production of formation fluids. Additional problems can result, such as the tendency of proppant to become dislodged from the fracture, resulting in an undesirable closing and decreased efficiency of the fracturing operation.
Optimally, the fracturing gel will begin to break when the pumping operations are concluded. For practical purposes, the gel should be completely broken within a specific period of time after completion of the fracturing period. At higher temperatures, for example, about 24 hours is sufficient. A completely broken gel will be taken to mean one that can be flushed from the formation by the flowing formation fluids or that can be recovered by a swabbing operation.
By comparison, certain gels, such as those based upon guar polymers, undergo a natural break without the intervention of chemical additives; however, the break time can be excessively long. Accordingly, to decrease the break time of gels used in fracturing, chemical agents are incorporated into the gel and become a part of the gel itself. Typically, these agents are either oxidants or enzymes that operate to degrade the polymeric gel structure.
Still, obtaining controlled breaks using various chemical agents, such as oxidants or enzymes, is challenging. Oxidants are ineffective at low temperature ranges from ambient temperature to 130° F. They require either higher temperatures to function as breakers or a coreactant to initiate cleavage of the viscosifying polymer. Oxidants do not necessarily break the polysaccharide backbone into monosaccharide units. The breaks are nonspecific, creating a mixture of macromolecules. Further, common oxidants are difficult to control because they not only attack the polymer, but they also react with any other molecule that is prone to oxidation. For example, oxidants can react with the tubing and the linings in the well, or the resins on resin-coated proppants.
In contrast, enzymes, are catalytic and substrate specific, hydrolyzing distinct bonds of the polymer. As a catalyst, they will hydrolyze many polymeric bonds before they eventually degrade. Enzymes can avoid the high temperatures associated with the chemical oxidants. However, they often operate under narrow pH and temperature ranges.
Conventional enzymes used to degrade galactomannans have maximum catalytic activity under mildly acidic to neutral conditions (pH 5 to 7). Activity profiles indicate the enzymes retain little to no activity at higher pH values. Their activity rapidly declines above pH 8, and they begin to denature above pH 9. This can pose a problem, for example, in borate-crosslinked guar gels, because these gels are also pH dependant, generally needing a pH in excess of 8 to initiate the gellation. As the pH increases, the resulting gel becomes stronger. Normally, when enzymes are used with borate-crosslinked gels, the gels are buffered to maintain a pH range of 8.2 to 8.5 to ensure gellation, but to inhibit enzyme degradation. This technique requires high concentrations of both borate and enzyme. Unfortunately, while ensuring good breaks, the initial gel stability and proppant transport capability is weakened. The determination of the optimum enzyme concentration is a compromise between initial gel stability and an adequate break.
Moreover, these enzymes work well at ambient to moderate temperatures (75° F. to 150° F.). At elevated temperatures, (>150° F.) they quickly denature and lose activity. The galactomannans used in conventional enzyme formulations have a temperature maximum of approximately 150° F. Activity profiles indicate that the enzyme retains little to no activity past this point, while many downhole fracturing operations are conducted at temperatures in excess of 150° F.
It would be an important advantage for the well operator to have greater control over the enzyme activity and, hence, the rate of hydrolysis of the polymer, to allow for a more timely and adequate break. Control over the enzyme activity could also allow the well operator to better work within the constraints of pH and temperature, and even allowing for less enzyme to be used fracturing fluid formulations.